A flip chip is generally a monolithic semiconductor device, such as an integrated circuit (IC), having bead-like terminals formed on one of its surfaces. The terminals, usually in the form of solder bumps, serve to both secure the flip chip to a circuit board and electrically interconnect the chip circuitry to a conductor pattern formed on the circuit board. Flip chip technology is compatible with a variety of circuit board types, including ceramic substrates, printed wiring boards, flexible circuits, and silicon substrates. The solder bumps are typically located at the perimeter of the flip chip on electrically conductive bond pads that are electrically interconnected with the circuitry on the flip chip. Due to the numerous functions typically performed by the microcircuitry of a flip chip, a relatively large number of solder bumps are often required. The size of a flip chip is typically on the order of about thirteen millimeters per side, resulting in the solder bumps being crowded along the perimeter of the flip chip. As a result, flip chip conductor patterns are typically composed of numerous individual conductors that are often spaced apart about 0.1 millimeter or less.
Flip chips are widely used in the electronics industry as a result of their compact size and their characteristic of being directly attached to substrates without additional packaging. Another process for directly attaching an IC device to a substrate is by the wire bonding process. Such an IC device has a number of bond pads that are wire bonded to bond pads of a complementary conductor pattern on the substrate to which the device is being attached. The bond pads on the IC device are typically aluminum or an aluminum-base alloy for various known processing and performance-related reasons. The wire is often gold, which will bond well with the aluminum bond pad if the bonding operation is properly performed.
Though wire-bonded ICs are widely used, flip chips are generally smaller, less expensive to mount, and more versatile, being suitable for a wider variety of electronic products than are chip-and-wire ICs. Consequently, there has evolved a demand for flip chip bumping and attachment of surface-mount devices that were originally designed for attachment by wire bonding. Several alternatives have been contemplated for this conversion process. One example is illustrated in FIGS. 1A through 1C, which show an IC device 10 on which a wire bond pad 12 has been formed. The bond pad 12 is conventionally formed of aluminum or an aluminum-base alloy, and is therefore susceptible to corrosion if left exposed. Consequently, a passivation layer 16 overlies the surface of the device 10, with a square-shaped opening 14 being formed in the passivation layer 16 to expose an interior region of the bond pad 12. However, the exposed region of the bond pad 12 is too large for forming a solder bump for flip-chip mounting the device 10. In particular, any attempt to form a solder bump on the exposed region of the bond pad 12 would yield a solder bump having inadequate height and a tendency to short with adjacent solder bumps.
Therefore, the process of FIGS. 1A-1C further entails depositing a second passivation layer 18 on the bond pad 12 and the original passivation layer 16, and then developing a circular-shaped opening 20 in the second passivation layer 18, as shown in FIG. 1B. This step generally can be performed by spinning an organic dielectric material on the substrate 10, photolithographically developing the opening 20 using known methods, and then curing the dielectric material. The opening 20 shown in FIG. 1B is sized and shaped to enable the deposition of a controlled amount of solderable material 22 on that portion of the bond pad 12 re-exposed by the opening 20, the result of which is illustrated in FIG. 1C. The solderable material 22 forms a bond pad that enables solder to be deposited and reflowed to form a suitable solder bump (not shown).
Though the process represented in FIG. 1A through 1C yields a suitable bond pad for a flip chip solder bump, the requirement for an additional passivation layer 18 and a masking operation to form the opening 20 represent a significant impact on processing costs and scheduling.
In view of the above, it would be desirable if a process were available that enabled wire bond pads to be converted to flip chip solder bump pads, but that avoided the cost and processing disadvantages of additional masking operations.